1. Field of the Invention
This invention relates to a wavelet encoding method and apparatus and a wavelet decoding method and apparatus. More particularly, it relates to wavelet encoding employing tile-based wavelet transform for picture compression/expansion.
2. Description of the Related Art
Among conventional representative picture compression systems, there is a Joint Photographic Coding Experts Group (JPEG) standardized by the International Organization for Standardization (ISO). This JPEG system is the system for compression-encoding mainly still pictures using the discrete cosine transform (DCT) and is known to give an optimum encoded or decoded picture subject to use of a larger number of allocated bits. However, if, in this system, the number of encoding bits is reduced to a certain extent, block distortion proper to DCT becomes outstanding to render subjective deterioration apparent.
On the other hand, investigations in a system of splitting an image into plural bands by a filter combined from high-pass and low-pass filters, termed a filter bank, and of encoding the resulting signals from band to band, are going on currently. In particular, the wavelet transform is retained to be a promising new technique to take the place of DCT since the wavelet transform is free from the inherent defect of DCT that the block distortion becomes outstanding in case of high compression.
Nowadays, the JPEG system, MPEG (Moving Picture Image Coding Experts Group), or the digital video system is used in a major proportion of electronic products, such as electronic still cameras or video movies. These compression systems use DCT as the transform system. In near future, wavelet transform based products of the above type will make their debut on the market. Also, investigations are going on briskly in various research institutions for improving the efficiency of the encoding system. In fact, in JPEG2000, which is currently worked on by ISO/IEC/JTCI SC29/WG1, as the same organization as JPEG, and which is thought to be a promising next-generation international still picture standard system, as a successor to JPEG, wavelet transform is anticipated to be used in preference to the pre-existing DCT of JPEG as a basic transform system for picture compression.
In the above-described wavelet transform, it is usually necessary to apply wavelet transform to the entire picture and to store the produced wavelet transform coefficients transiently in a memory. Thus, if the picture size is larger, an extremely large memory capacity is required, which is inconvenient for an apparatus with a limited memory capacity, such as an electronic still camera, video camera recorder or a so-called personal digital assistant (PDA).
It is therefore an object of the present invention to provide a wavelet encoding method and apparatus and a wavelet decoding method and apparatus in which the size of the memory capacity, retained to be problematical in effecting high efficiency compression by wavelet transform, can be decreased, and which is able to realize high compression efficiency.
In one aspect, the present invention provides an wavelet encoding apparatus including means for splitting an input picture into a plurality of tile pictures, means for applying wavelet transform from one tile picture to another, means for scanning wavelet transform coefficients, means for quantizing wavelet transform coefficients, quantizing as-scanned quantization coefficients, and means for entropy encoding the quantization coefficients to output an encoded bitstream, wherein the wavelet transform means includes means for symmetrically expanding pixels in a given tile picture within a range outside the tile picture influenced by filtering, and for executing convolution.
In another aspect, the present invention provides a wavelet encoding method comprising the steps of splitting an input picture into a plurality of tile pictures, applying wavelet transform to the input picture from one tile picture to another, scanning wavelet transform coefficients, quantizing as-scanned wavelet transform coefficients and entropy encoding the quantization coefficients to output an encoded bitstream, wherein the wavelet transform step includes a step of symmetrically expanding pixels in a given tile picture to a range outside the tile picture influenced by filtering, and of executing convolution.
In still another aspect, the present invention provides a wavelet decoding apparatus including means for being fed with or reading out an encoded bitstream for entropy decoding the encoded bitstream, means for dequantizing produced quantization coefficients, means for back-scanning the produced quantization coefficients to restore the original sequence of coefficients, means for inverse wavelet transforming back-scanned coefficients to generate tile pictures and means for synthesizing produced tile pictures to furnish an ultimate output picture.
In still another aspect, the present invention provides a wavelet decoding method comprising the steps of being fed with or reading out an encoded bitstream for entropy decoding the encoded bitstream, dequantizing produced quantization coefficients, back-scanning the produced quantization coefficients to restore the original sequence of coefficients, inverse wavelet transforming back-scanned coefficients to generate tile pictures and synthesizing produced tile pictures to furnish an ultimate output picture. In yet another aspect, the present invention provides a wavelet encoding/decoding apparatus for splitting an input picture into a plurality of tile pictures, wavelet encoding the input picture from one tile picture to another to generate an encoded bitstream and for wavelet decoding the encoded bitstream. The wavelet transform means includes means for symmetrically expanding and convolving wavelet transform coefficients in the inside of a given tile picture to a range outside the tile picture affected by filtering, there being no overlapping area between the pre-set tile picture and neighboring tile picture(s). The inverse wavelet transform means, as counterpart means of the wavelet transform means, includes means for symmetrically expanding and convolving the wavelet transform coefficients in the inside of the pre-set tile picture or means for setting the wavelet transform coefficients in a range outside the pre-set tile picture affected by filtering all to 0 and for effecting convolution.
In the tile picture wavelet encoding method and apparatus according to the present invention, an input picture is split into plural tile pictures and wavelet transform is applied from one tile picture to another. The wavelet transform coefficients then are scanned, and the as-scanned coefficients are entropy-encoded to output an encoded bitstream. Also, in the wavelet transform, pixels on an outer periphery of the tile picture are symmetrically expanded and convolved, thus realizing encoding/decoding with significant memory space reduction as compared to a system in which wavelet transform is applied to the entire picture. Also, there is no constraint condition concerning the number of wavelet splitting which has posed problems in overlap tile-based wavelet encoding, thus realizing a high encoding efficiency at all times.
Moreover, if it is desired to decode only a specified tile picture, decoding can be realized completely independently of neighboring tile(s) by combining wavelet transform means applying symmetrical pixel expansion and convolution with inverse wavelet transform means applying counterpart symmetrical wavelet transform coefficient expansion and convolution.
If wavelet transform means employing integer precision wavelet filter is used, inverse wavelet transform means not accompanied by overlap is used, so that there is no necessity of adding rounding error at the time of filtering and hence the distortion in the vicinity of the tile picture is not apparent.
In the present wavelet decoding method and apparatus, an encoded bitstream is inputted or read out and entropy-decoded to produce quantization coefficients, which then are dequantized to produce transform coefficients. These transform coefficients are back-scanned to restore the sequence of original coefficients. The back-scanned coefficients are inverse wavelet transformed to generate tile pictures, which then are synthesized to furnish an ultimate output picture. After inverse wavelet transform, wavelet transform coefficients in the inside of the tile picture are symmetrically expanded and convolved. Alternatively, wavelet transform coefficients outside the tile pictures affected by filtering are all set to 0 and convolved to enable inverse transform and decoding independently of neighboring tile pictures. This gives a desirable result that deterioration in the boundary portions between neighboring tile pictures can hardly be perceived in a higher bitrate (low compression ratio). It should be noted that linear convolution has a desirable feature that smooth junction can be achieved in the boundary portions between neighboring tile pictures under the effect of linear interpolation by wavelet filter which may be achieved by setting the coefficients in the boundary portions to zero. Also, in the inverse wavelet transform means, since there is no necessity of reading out wavelet transform coefficients of the neighboring tile pictures in an overlapped fashion, a given tile picture can be inverse-transformed and decoded independently of the neighboring tile pictures.
Thus, according to the present invention, high efficiency encoding and high quality decoding can be achieved since both the features of high quality under high compression and partial tile picture decoding can be achieved and wavelet transform means can be matched to inverse wavelet transform means.